Rechargeable batteries, such as lithium-ion rechargeable batteries, are widely used as electrical power for battery-powered portable electronic devices, such as cellular telephones, portable computers, camcorders, digital cameras, PDAs and the like. A typical lithium-ion battery pack for such portable electronic devices employs multiple cells that are configured in parallel and in series. For example, a lithium-ion battery pack may include several blocks connected in series where each block includes one or more cells connected in parallel. Each block typically has an electronic control that monitors voltage levels of the block. In an ideal configuration, each of the cells included in the battery pack is identical. However, when cells are aged and cycled, cells tend to deviate from the initial ideal conditions, resulting in an unbalanced cell pack (e.g., unidentical capacity, impedance, discharge and charge rate). This unbalance among the cells may cause over-charge or over-discharge during normal operation of the rechargeable batteries, and in turn can impose safety concerns, such as explosion (i.e., rapid gas release and possibility for fire).
Traditionally, the conventional lithium-ion rechargeable batteries have employed LiCoO2-type materials as the active component of lithium-ion battery cathodes. For such a lithium-ion cell employing LiCoO2-type active cathode materials to be fully charged, the charge voltage is usually 4.20V. With lower charging voltage, the capacity is lower, which corresponds to lower utilization of active LiCoO2 materials. On the other hand, with higher charging voltage, the cell is less safe. In general, it is a challenge for LiCoO2-based lithium-ion cells to have a high capacity, for example higher than about 3 Ah due to a high safety concern. In order to minimize the safety concern, lowering the charge voltage is one option. However, this will lower the cell capacity, and in turn lower cell energy density. To obtain high capacity, increasing the number of cells in one battery pack may be another option rather than increasing the charge voltage. However, the increase in the number of cells can result in increased probability of unbalance among the cells, which can cause over-charge or over-discharge during normal operation, as discussed above.
The largest mainstream cell that is typically used in the industry currently is a so-called “18650” cell. This cell has an outer diameter of about 18 mm and a length of 65 mm. Typically, the 18650 cell utilizes LiCoO2 and has a capacity between 1800 mAh and 2400 mAh but cells as high as 2600 mAh are currently being used. It is generally believed that it is not safe to use LiCoO2 in a larger cell than the 18650 cell because of a safety concern associated with LiCoO2. Other cells larger than the 18650 cells exist in the art, for example, “26650” cells having an outer diameter of about 26 mm and a length of 65 mm. The 26650 cells typically do not contain LiCoO2 and have worse performance characteristics in terms of Wh/kg and Wh/L than the 18650 cells employing LiCoO2.
Therefore, there is a need to develop new active cathode materials for lithium-ion batteries that minimize or overcome the above-mentioned problems. In particular, there is a need to develop new active cathode materials that can enable manufacturing large batteries, for example, larger than the conventional LiCoO2-based batteries (e.g., 18650 cells) in volume and/or Ah/cell.